PROJECT 2 - PROJECT SUMMARY/ABSTRACT As estimates predict that AD is likely to become one of the most important global public health issues by 2050, the urgency to develop mechanism-based disease modifying therapies for AD is ever increasing. Over the past two decades, animal model systems have been instrumental in clarifying the molecular mechanisms and testing therapeutic strategies for AD. Notwithstanding advances made using current rodent models that exhibit robust A amyloidosis and tau pathology, success in translating preclinical drug discoveries to the clinic remains rather disappointing. A major limitation of current mouse models is the lack of robust age-dependent neuronal loss induced by the interaction of amyloid and tau aggregates in their brains, salient neuropathological hallmarks of individuals with AD. To overcome this problem, we propose to characterize a recently created mouse model that not only exhibits A amyloidosis and tauopathy, but also displays robust age-dependent neuronal loss. We will use our new mouse model to test our overarching hypothesis that A induces tau aggregation to initiate synaptic dysfunction and promote neuronal loss. We believe that such a model system will provide the opportunity for the first time to evaluate A amyloidosis and tauopathy dependent loss of neurons as the principal outcome measure in preclinical testing of mechanism-based therapies, such as those designed to target tauopathy and/or A amyloidosis. Toward this goal, we recently generated lines of transgenic mice expressing human four-repeat tau fragment containing the DK280 mutation (Tau-4RDK280) in the forebrain that not only developed AD-like tau aggregates but displayed memory deficits, age-dependent neuronal loss and brain atrophy. By crossbreeding Tau4R-DK280 mice with a model of A amyloidosis, APPswe;PS1DE9 mice28, we generated mice that develop both Ab and tau pathologies (Tau608-AP mice). Significantly, both the tauopathy and cell death observed in Tau4R-DK280 mice were greatly accelerated in Tau608-AP mice, which support a model that the presence of A could accelerate the development of AD-like tau pathologies and age-dependent neuronal loss. Our proposal thus will allow us to focus attention on several important questions. First, it is not completely clear how accumulation of tau aggregations leads to neuronal loss. Second, it is not known whether and how A interacts with tau aggregates to promote the early pathological alterations that leads to synaptic dysfunction and accelerates neuronal loss. Finally, it remains an open question as to whether early interventions designed to target tau will attenuate neuronal loss in the presence of amyloid pathology in AD. In Project 2, we will use our novel mouse models to directly address these important questions. Our proposed studies will identify the mechanisms underlying the interactions between tau and A pathology, and provide valuable information to evaluate the effectiveness of targeting tau pathologies.